Colonic CX3CR1+ macrophages
play a crucial role in the anti-inflammatory effect of LMT503 in
DSS-induced colitis
Although the ratio and absolute numbers of colon macrophage populations
were not altered, LMT503 treatment resulted in polarization from
inflammatory to an anti-inflammatory character. It is known that colon
resident macrophages show high expression of CX3CR1.
During colitis, Ly6C+ monocyte infiltration to the
lamina propria can differentiate into
CX3CR1int macrophages which are
pro-inflammatory. However, afterwards, further maturation into
CX3CR1hi regulatory macrophages drives
resolution of inflammation (Bain et al., 2013). These gut
CX3CR1hi macrophages can clear out
apoptotic neutrophils and support tissue healing (Arandjelovic &
Ravichandran, 2015; Castoldi et al., 2023). Thus,
CX3CR1+ macrophages are crucial in
maintaining gut homeostasis. They play a key role in IBD. To verify that
macrophage character shift by LMT503 treatment contributed to
DSS-induced colitis alleviation, we used a
CX3CR1+ macrophage depletion system.
For depletion of CX3CR1+ cells,
CX3CR1-DTR mice were i.p. injected with
diphtheria toxin during DSS-induced colitis. Under DSS treatment, mice
deficient of CX3CR1+ macrophages
showed body weight loss and high score of disease activity index,
similar to those of WT mice (Figure 6a,b). However, absence of
CX3CR1+ macrophages abrogated
alleviation of colitis including less body weight loss, low score of
disease activity index, and recovered colon length after LMT503
treatment. At day 10, group comparison of body weight changes, disease
activity index, and colon length suggested that the effect of LMT503
treatment could be abolished in absence of
CX3CR1+ macrophages (Figure 6c-e).
After CX3CR1+ macrophage depletion,
LMT503 could not control pro-inflammatory cytokines IL-6 (Figure 6f).
LMT503 treatment significantly increased IL-10 and IL-22 production in
inflamed colon which could be diminished after depletion of
CX3CR1+ macrophages (Figure 6g). These
data suggest that LMT503 can exert its effect through targeted
modulation of CX3CR1+ macrophages to
alleviate DSS-induced colitis.